AEC is a mode of operation of an X-ray machine by which the x-ray output is automatically controlled and terminated when a pre-set radiation exposure to the image receptor is reached. It may also operate in such a way that a pre-exposure is made before the diagnostic exposure and a sensor, e.g. part of the sensor used for the diagnostic imaging, is measuring the outcome of the pre-exposure and from the measured data the optimum parameters for the diagnostic exposure is calculated. The parameters that can be optimised for a diagnostic exposure are typically x-ray tube potential (kV), x-ray tube current (mA), x-ray exposure time, x-ray filter material, and x-ray anode target material may also be automatically selected.
The function of the AEC is to assure that the image is exposed correctly, independent of the object. A correct exposure implies that the image receives sufficient statistics. The level of statistics in an image is proportional to the tube current (mA) and the exposure time (s). Thus, the tube loading (mAs) is a quantity used to describe the exposure.
The optical density control of a film system is essential in order to expose modern high-sensitive films correctly. A digital system, e.g. for mammography, could similarly struggle to obtain a certain signal to noise ratio (SNR) in the image for different breast types and entrance spectrum.
There are a number of ways to implement an AEC. The patent documentation describes some, e.g.:
U.S. Pat. No. 4,357,708 relates to an arrangement to the path of movement of the photographic exposure unit comprised of an x-ray tube and image layer (or film) carrier can be selected. For determination of the photographic exposure time, either a mAs-relay or an automatic exposure timer with a radiation detector may be placed in control of the energization of the x-ray tube. The x-ray tube current or the current of the radiation detector are integrated and the integrator contents are sampled at a predetermined time following commencement of the photographic exposure. The sampled value is compared with a reference value and the running speed of the photographic exposure unit is influenced in such a fashion that the photographic exposure time determined by the switching of the mAs-relay or by the automatic exposure timer approximately corresponds to the running time of the photographic exposure unit.
In U.S. Pat. No. 4,383,327 a scanning radiographic system employing a multi-linear array is disclosed. The system includes a source of electronic radiation, which is incident upon the multi-linear array. The multi-linear array includes radiation sensors each of which is adapted to generate an intensity signal as a function of the amount of radiation sensed thereby. Each sensor has associated therewith a means for holding or storing its respective intensity signals. The intensity signals thus held may be continually up-dated to reflect subsequent intensity signals resulting from additional radiation sensed by the respective sensors. An opaque object to be scanned by the radiographic system passes through the beam of radiation in a controlled fashion. This controlled motion is synchronized and coordinated with the shifting of the up-dated intensity signals so that the speed and course of travel of a particular up-dated intensity signal through the holding means of a given group of said sensors is optically aligned with the speed and course of travel of the radiation passing through a given area of the opaque specimen. In this fashion, there is generated one up-dated intensity signal corresponding to a given area of the opaque specimen. These up-dated intensity signals are then collected and processed by a suitable visual system.
WO 03/043497 relates to automatic exposure control implemented in imaging by electromagnetic radiation, in particular to automatic exposure control in film-based mammography, which is based on a completely new approach as compared with the solutions currently in use, which utilize adjustment curves and/or tables constructed on the basis of empiric tests. The new approach includes modelling into the AEC system the radiation spectra obtainable from the radiation source as a function of its operating parameters and attenuation of the spectrum as the radiation traverses components of the imaging apparatus. By measuring the thickness of the object to be imaged and knowing the initial spectrum and its calculable behaviour, a correspondence between the AEC signal and the desired darkening of the image can be achieved which is based on true density of the object being imaged.
Other papers include “A scanning system for chest radiography with regional exposure control: theoretical considerations”, Plewes D B, Med Phys. 1983 September-October; 10(5): 646-54. According to this document, the presence of scattered radiation and the small useful exposure range of radiographic film limit conventional chest radiography. A computer-assisted scanning system to minimize these two effects is outlined. The system uses a small beam of radiation swept over the patient's chest in a raster pattern to expose a conventional film cassette, while a slit collimator scanning between the patient and the film serves to reject scattered photons. A microcomputer measures beam attenuation by the patient with a detector placed behind the film, which in turn automatically adjusts the x-ray tube output to minimize excursions in film exposure as the beam scans. A formalism, which relates the patient transmission and film exposure distribution, is developed and a system transfer function is given. It is shown that such a system operates as a spatial filter, which attenuates film contrast for structures of spatial frequency less than the inverse scanning beam width. By manipulating the software parameters of the feedback network, it is possible to alter this filter and produce radiographs with low spatial frequency enhancement, attenuation, or contrast inversion.
US 2003/0174806 relates to an apparatus for recording a two-dimensional image of an object. The apparatus comprises a plurality of one-dimensional detector units, each exposed to ionizing radiation transmitted through or scattered off the object, which is arranged for one-dimensional imaging of the radiation. The apparatus includes a device for moving the detector units relative the object while the detector units repeatedly detect to create the two-dimensional image of the object and a control device for controlling the detector units to detect ionizing radiation during a short period of time before or during an initial part of the movement, calculating an optimum exposure time for the repeated detection based on the short period of time detection, and controlling the repeated detection to automatically obtain the optimum exposure time.
According to this document, a pre-scan is performed before conducting a examination scan. The pre-scan values are used to adjust the examination scan parameters, such as scan speed.
In U.S. Pat. No. 5,585,638 an x-ray detector for an automatic exposure control system is disclosed. The apparatus has a substrate of carbon composite material with a first layer of conductive material on a major surface of the substrate and a second layer of homogeneous semiconductive material is deposited on the first layer and has an electrical characteristic, such as conductivity, that varies in response to impingement of x-rays. A third layer of conductive material is formed on the surface of the semiconductor layer and is divided into a plurality of electrode elements which define a plurality of regions in the layer of semiconductive material. By sensing the conductivity between the first layer and each of the electrode elements, the intensity of x-rays striking the different regions can be measured. Thus, a scintillator is used to convert protons to photons.
The detector according to this invention converts protons to photons and then to electrical signals. However, the system does not use photon counting, i.e. the number of the photons incident to a detector.